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In 1798 it was purchased, along with a thousand acres of lawns, gardens, and plantations of ancient trees, by a successful banker and MP, Henry Peters (see his post on the “People” page of this site). Over the three decades of his ownership, Peters expanded the property and added many outbuildings, some at the direction of fashionable London architect Sir John Soane, whose house is now one of the more eccentric museums in the metropolis. (There exist designs for a lodge, dairy, ice house, greenhouse, and stables, and some of these buildings yet stand.) Soane also made extensive improvements to the house’s interior. Peters raised twelve children there.
Writing in 1822, John Timbs gives us the best description we have of the Betchworth Castle estate in the Peters years: ‘The castle is seated on an eminence, on the western bank of the river Mole. It has a handsome and venerable appearance, for which it is greatly indebted to the taste of its present owner, who has made considerable improvements in the buildings and grounds; and has enlarged the estate by extensive purchases. . . . Environed on all sides with lawns, tastefully-arranged gardens, shrubbery-walks, and banks thickly clothed with luxuriant evergreens, it forms one of the most enchanting retreats of this country. The detached offices of the establishment exhibit great taste in their constructions; the style of rural architecture rendering them both useful and ornamental appendages. . . . The park is remarkable for the stately timber with which it is adorned. Approaching the castle from Dorking, the road leads through an outer park, skirted with rows of old chestnut-trees, of large dimensions; and the inner, at the extremity of which the castle is situated, has two fine avenues, the one of elms, and the other, 850 yards in length, composed of a triple row of limes [lindens] of gigantic size and height. The last avenue resembles the nave of a cathedral: the trees form on the outside a vast screen or wall of verdure; within, the branches, meeting at a great height in the air from the opposite rows, form Gothic arches, and exclude every ray of the meridian sun. The river Mole, washing the verdant edge of the park, has in some parts an important breadth, and is thickly shaded with aquatic trees and bushes’ (A Picturesque Promenade, pp. 217–18).

Sadly, all of Peters’s investments were soon to go to waste. In 1834 the property was bought by Henry Thomas Hope, the even richer owner of The Deepdene nearby. Hope plundered much of the castle to use its building materials in his expansion of The Deepdene’s house, leaving the remainder of the castle to fall into ruin.
The ruins still exist today, heavily decayed and unstable, tenanted in rumour only by ghosts and standing as a monument to the folly of those whose vanity impels them to spend fortunes on the aggrandisement of their surroundings. In Coldharbour Gentlemen Henry Peters ruefully acknowledges this folly to the young hero—but he committed it anyway.

14 September 2021
Sources
The Paranormal Database, ‘Lord Hope’.
John Timbs, A Picturesque Promenade round Dorking, in Surrey (London, 1822).
The Victoria History of the County of Surrey, edited by H. E. Malden, vol. III (University of London, repr. 1967).